A need has been felt for a long time to render metal articles such as pieces of jewelry, for example, watch-cases, scratch- and corrosion resistant so that they retain their original appearance for a maximum length of time. It has been attempted, for example, to achieve superior scratch resistance by chromium-plating of watch-cases. However, a decisive improvement of these properties was not attained until cemented carbides were used in watch cases. Once the chemical and mechanical resistance had been attained, it was desired to give these watch-cases a more attractive appearance. This was achieved in part by making the watch-cases wholly of a metallic hard material having a gold-like color, as, for example, titanium nitride, tantalum nitride, zirconium nitride, etc., or the watch-cases were coated with colored hard materials, for example, the yellow titanium nitride or red titanium carbonitride. With this method, a single layer of a certain color was always deposited on the metallic substrate. Methods are also known today to equip metallic articles such as pieces of jewelry or watch cases with a decorative surface design formed of differently colored zones. A known method for the production of such a decorative design consists in coating the surface of the base body with metals such as titanium, tantalum, tungsten, vanadium, zirconium or hafnium and oxidizing different zones under different conditions. With suitable choice of these conditions, differently colored zones are produced. However, the coatings produced by this method have a low scratch and wear resistance. The present invention is intended to overcome these deficiencies.
It is a primary object of this invention to provide an article of jewelry, for example, a watch-case, having differently colored surface zones, wherein the differently colored surface zones are formed by hard materials of different chemical composition. As such hard materials, carbides, carbonitrides, nitrides, silicides or borides of the Group III to VI metals of the periodic table according to Mendeleef may be used, singly or in the form of solid solutions or mixtures, where metallic as well as non-metallic elements may be present.